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<TITLE>80386 Programmer's Reference Manual -- Opcode CBW</TITLE>
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<H1>CBW/CWDE -- Convert Byte to Word/Convert Word to Doubleword</H1>

<PRE>
Opcode    Instruction     Clocks          Description

98        CBW             3               AX := sign-extend of AL
98        CWDE            3               EAX := sign-extend of AX
</PRE>

<H2>Operation</H2>

<PRE>
IF OperandSize = 16 (* instruction = CBW *)
THEN AX := SignExtend(AL);
ELSE (* OperandSize = 32, instruction = CWDE *)
   EAX := SignExtend(AX);
FI;
</PRE>

<H2>Description</H2>

CBW converts the signed byte in AL to a signed word in AX by extending the
most significant bit of AL (the sign bit) into all of the bits of AH. CWDE
converts the signed word in AX to a doubleword in EAX by extending the most
significant bit of AX into the two most significant bytes of EAX. Note that
CWDE is different from CWD. CWD uses DX:AX rather than EAX as a destination.

<H2>Flags Affected</H2>

None

<H2>Protected Mode Exceptions</H2>

None

<H2>Real Address Mode Exceptions</H2>

None

<H2>Virtual 8086 Mode Exceptions</H2>

None


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Chapter 17 -- 80386 Instruction Set</A><BR>
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